WP e-commerce: Inventory not working

Hi! The inventory feature (number of items on stock) sometimes work, sometimes does not. When I get an order, I go to Store Sales, open the order and then click “Close Order”. Most of the time the inventory changes (minus 1 for the sold product) but sometimes it does not work! I am so frustrated now because I already had to email a few customers to tell them that the site had a glitch and I sold them an out of stock item. PLEASE HELP!!!